This invention relates generally to the sensing of temperatures in occupied spaces and deals more particularly with an electronic temperature sensor which is constructed and arranged to be mounted on a wall substantially flush with the wall surface.
In office buildings and other occupied spaces which are serviced by heating, ventilating and air conditioning (HVAC) systems, the HVAC equipment is normally controlled by a temperature sensor or thermostat in order to maintain the temperature in the occupied space at the desired level. Typically, the temperature sensor is contained in an enclosure which protrudes into the room from one of its walls. It is also common for the sensor to be mounted in a return air opening or duct where there is a positive flow of air from the conditioned space around the temperature sensing element. In either case, accuracy in the sensing of the temperature requires that the air in the room have good thermal contact with the temperature sensing element. Also, the element itself should have the minimum possible thermal capacity compared to its thermal conductivity to the room air so that it can respond quickly to temperature changes.
In the case of wall mounted sensors or thermostats, the active component is normally hidden behind a cosmetic cover which is provided with openings so that room air can migrate behind the cover and come into contact with the sensing element. Accuracy requires that the element have maximum thermal contact with the air in the conditioned space and minimum thermal contact with the wall which may have a temperature considerably different from that of the room air. Therefore, it is standard practice to mount the thermostat unit directly on the wall surface rather than recessing it or mounting it flush with the wall. Even though units which protrude into the room are recognized as being architecturally and aesthetically undesirable, they have largely been viewed as a necessary evil which must be tolerated in order to achieve both good thermal contact with the air and minimal thermal contact with the wall.
In mechanical or electrical sensing devices, the room air is allowed to directly contact a bimetal strip which provides the switching action for the HVAC equipment. The bimetal strip is thermally isolated from the remainder of the structure to the extent possible in order to prevent the wall temperature from effecting the temperature measurement. In electronic temperature sensors, the room air contacts a low power temperature sensitive element such as a thermocouple or a temperature sensitive resistor. The voltage or resistance of the element is measured from a remote location where more sophisticated active circuitry is provided.
The need to physically separate the active electronic circuitry from the sensing element creates calibration problems when the sensing element is initially installed or replaced. The element and wiring must be matched to the active circuitry which measures the output signal from the element. Although precalibrated sensing elements are available, they are relatively expensive and must be field calibrated in order to take into account the unknown resistance in the wiring which leads from the sensing element to the active circuitry. Field calibration adds significantly to the time and labor necessary to install the device and increases the cost accordingly. Another problem is that possible electromagnetic noise in the environment creates an additional uncertainty, particularly when the signal level is relatively low.
In recent years, small integrated circuits have been developed for use as low power, temperature sensitive current sources, as exemplified by the device shown in U.S. Pat. No. 4,123,698. The patented device is precalibrated on an absolute temperature scale (1 micro amp/.degree.K., for example). Because the device acts as a current source, its output can be measured at a remote location without being distorted by the resistance of the wiring which transmits the signal to the remote location. However, the signal level is so low that electromagnetic interference is a significant problem that can adversely effect the accuracy of the temperature measurement. Also, circuits of this type have a wide span in their intrinsic temperature range, and this makes their accuracy unacceptable in the relatively narrow temperature range of human comfort (22.degree.-26.degree. C.).
These problems have been recognized, and circuits have been proposed for converting the low power level and wide temperature range to a higher power (such as 4-20 milliamps) and a narrower temperature range (such as 17.degree.-30.degree. C.). Although these circuits have rather high power requirements in comparison to the low power sensing devices, they can be used successfully in applications where there is good thermal contact with the medium which is being sensed, such as when the sensor is embedded in solid machinery or is immersed in liquid.
Similar circuits have been proposed for use in the sensing of air temperatures in occupied spaces, but the results have not been entirely satisfactory. When the sensor can be conveniently mounted in a moving air stream such as in a return air duct, the heat generating components are not particularly objectionable because the air stream acts to dissipate the heat. However, attempts to mount the temperature sensor and the heat generating active components in a wall mounted enclosure have not met with success. In order to obtain the necessary cooling from the convection of room air, it is necessary for the enclosure to protrude out well into the room, and this is undesirable from an architectural and aesthetic standpoint. Even then, satisfactory operation requires minimum convection which is not always present and ma fluctuate in any event. Due to the presence of the heat generating components, there is a noticeable offset (typically on the order of 1.degree. C.), between the actual room temperature and the measured temperature. In order to account for this offset, the device is usually calibrated low intentionally. However, if the actual convection is different from the convection that is expected when the device is calibrated, the temperature signal is inaccurate.
At times, convection of air in the vicinity of the temperature sensor can be completely or nearly completely stopped. Then, the temperature can build up in the dead air space in the device, and the cosmetic cover prevents the heat from being dissipated by radiation. In this situation, errors as large as several degrees centigrade can result, and temperature errors of this magnitude are unacceptable. In the HVAC industry, electronic temperature sensors are expected to perform more accurately than conventional sensors rather than being lower than average in accuracy as occurs with the electronic sensors that are currently available.
Two different approaches have been followed in attempting to improve the accuracy of electronic temperature sensors. First, convection has been promoted by enlarging the package which contains the sensor and mounting it such that it protrudes out well into the room. The aesthetic disadvantages of this type of arrangement make it unacceptable in most applications even if the accuracy is within acceptable limits. The other approach that has been used involves removing the active heat generating components from the vicinity of the temperature sensing element. This approach has its own disadvantages, most notably in the inconvenience, high cost and accuracy degradation associated with the need for field calibration to compensate for the resistance in the wiring.
Using the wall as a heat sink to receive the heat generated by the electronic components has been rejected because the wall temperature often differs appreciably from the air temperature which dominates human perception. If the device is embedded in the wall to remove the heat that is generated, the wall temperature significantly affects the sensing element and results in significant steady state temperature errors caused by the difference between the wall temperature and the air temperature in the occupied space. Even more importantly, thermal delays arise and the HVAC control system can be completely destabilized in its temperature response capabilities.